The principle objective of this project is to further characterize the protein isolated from rat liver that reversibly inhibits the proliferation of rat liver epithelial (RLE) cells at extremely low concentrations (ID50 = 0.2 ng/ml). The biological effects of this inhibitor protein (designated as LDGI) were compared with those of transforming growth factor-beta (TGF-beta) and recombinant tumor necrosis factor-alpha (rTNF-alpha) in a variety of normal and malignant cell culture systems. RLE cells were highly sensitive to the antiproliferative effects of LDGI and TGF-beta but were relatively insensitive to the cytostatic effects of rTNF-alpha. Aflatoxin B1 (AFB-1)-transformed RLE cells were resistant to the antiproliferative effects of TGF-beta and rTNF-alpha but showed sensitivity to the growth inhibitory effects of LDGI. Clones isolated from AFB-I-transformed RLE cells exhibited either sensitivity similar to parental AFB-Itransformed RLE cells or complete resistance to growth inhibition by LDGI. However, none of these clones exhibited sensitivity to the antiproliferative effects of TGF-beta or rTNF-alpha. Rat hepatoma (UVM7777) cells were insensitive to the cytostatic effects of all three growth modulators. Human breast carcinoma (MCF-7) and rat hepatoma (Rueber) cells were extremely sensitive to rTNF-alpha, exhibited low sensitivity to LDGI, but were resistant to the antiproliferative effects of TGF-beta. The rate of DNA synthesis in rat kidney fibroblasts (NRK) and human foreskin fibroblasts was stimulated two- to threefold in response to LDGI, TGF-beta and rTNF-alpha treatment. The growth rate of RLE cells transformed with a retrovirus containing both v-myc and v-raf (J2) was stimulated by both LDGI and TGF-beta.